Commercial processes for preparing 4,4''-dinitrostilbene-2,2'-dilsulfonic acid and the salts thereof generally consist in subjecting p-nitrotoluene-2-sulfonic acid salts to an oxidative condensation reaction in strongly alkaline medium.
Typically in such processes, a chemical oxidizing agent, such as oxygen or oxygen-enriched air (usually with a catalyst), sodium hypochlorite or chlorine, is employed in the presence of alkali or alkaline earth metal hydroxide compounds to result in the coupling of two molecules of p-nitrotoluenesulfonic acid salts to form one molecule of the 4,4'-dinitrostilbene-2,2'-disulfonic acid salt, usually via formation of a stable intermediate compound, 4,4'-dinitro-dibenzyl-2,2'-disulfonic acid salt.
However, conventional oxidizing processes are associated with various operating problems and expenses, e.g., safety hazards involved in the use of oxygen gas or chlorine solutions; or loss of reaction medium in effluent gas at the high oxygen sparging rates which are generally necessary. Additionally, the tendency for oxidative dye by-products to form may necessitate that numerous purification steps be carried out to isolate stilbene compound of sufficient purity for its predominant use as an intermediate in the synthesis of optical brightening agents and dyestuffs.
A particular problem associated with the known chemical oxidation processes concerns treatment and disposal of the large volumes of environmentally deleterious waste which are created, which in general comprise highly alkaline mixtures of hydrocarbon materials, including reaction by-products such as colored dyes and unreacted starting material, as well as metal compounds in high concentration Where chlorine is employed as the oxidizing agent, chlorinated hydrocarbons may also persist as an environmentally objectionable component. Extensive purification of the effluent, including steps of neutralization, removal of organics, and recovery of metal salts, is needed for compliance with environmental standards as well as to recover certain costly starting materials, e.g., compounds of alkali metals such as lithium.
It has therefore been an objective to provide a preparative process which overcomes certain of the limitations associated with the known oxidation processes, especially as these relate to effluent treatment and recovery of starting materials.